1. Technical Field
The present invention is directed to a system and method for operating a diesel engine in both conventional diesel and dual fuel operating modes, and more particularly to such a system and method in which a pulsed flow of diesel fuel is continuously introduced into the combustion chamber of the engine through a pilot injector during operation in both operating modes.
2. History of Related Art
Increasing concern over exhaust emissions regulation and fuel efficiency have led to an interest in burning a combustible gaseous mixture, such as propane or natural gas in diesel engines, particularly in diesel engines used in stationary, marine, and locomotive applications. Many of these applications use dual fuel combustion systems due to the requirement that diesel operation capability be retained, due to either limited refueling infrastructure, intermittent operation, or limited on-board fuel storage capability. Current dual fuel combustion systems are either homogeneous or heterogeneous gas-air charge systems, and each have distinct advantages and disadvantages.
Homogeneous charge duel fuel combustion systems typically require a reduction in compression ratio and/or a reduction in inlet air temperature to avoid undesirable early detonation of the gas-air charge. This problem is discussed in a Non-Petroleum Vehicular Fuels III Symposium paper The Diesel-Gas Dual-Fuel Engine, by G. B. O'Neal, Published by the Institute of Gas Technology, 1982. Natural gas composition and mixing of the gas-air charge are concerns with homogeneous charge dual-fuel systems, as they greatly affect the potential for detonation and combustion rate. Generally, either some form of load control, i.e., throttling or waste gate, is required for starting and/or light-load operation, or the engine operates on undiluted diesel fuel under those conditions. Such a dual fuel engine is described in an article titled The Dual Fuel Engine by G. A. Karim, appearing in Automotive Engine Alternatives, Robert L. Evans editor, Plenum Press, New York, 1987. Although diesel efficiency can be matched and significant reductions in oxides of nitrogen (NO.sub.x) emissions are obtained when operating in a homogeneous dual-fuel mode at rated conditions, the engine modifications required for dual-fuel operation cause substantial loss in efficiency when operating at reduced load or on 100% diesel fuel. Also, increased cold start and light load emissions occur, along with reduced efficiency, if the engine is unthrottled and operates on 100% diesel fuel at reduced compression ratios.
Heterogeneous, i.e., stratified, charge dual-fuel combustion systems typically feature direct injection of natural gas late in the compression cycle. By injecting natural gas at high pressure late in the cycle, premature detonation is avoided and no modifications to the inlet air and compression chamber are required. This operation is described in SAE Paper No. 872041, titled High-Pressure Late Cycle Direct Injection of Natural Gas in a Rail Medium Speed Diesel Engine, authored by J. F. Wakenell, G. D. O'Neal, and Q. A. Baker. Normal diesel operation is retained in the described engine operation without penalties in emissions or efficiency. In the dual-fuel operation mode, natural gas is stratified within the combustion chamber and burned in a manner similar to the diesel cycle. The dual-fuel efficiency is typically equivalent to, or better than, that of 100% diesel operation and NO.sub.x and particulate matter emissions are significantly reduced from typical diesel operating levels. Although the heterogeneous dual-fuel NO.sub.x and particulate emission reductions are less than homogeneous dual-fuel system levels, the improved efficiency at all operating conditions and reduced light-load emissions present a better trade-off in many applications.
While heterogeneous dual-fuel systems present many benefits over homogeneous systems, the expense and complexity of high pressure, direct-gas injection systems are prohibitive for many applications. Current direct gas injectors are electronically controlled and hydraulically or pneumatically actuated, requiring additional electronic controls and hydraulic or pneumatic systems for operation. Older, mechanical direct injection systems are much less flexible in control capability, and typically feature cam-actuated poppet valves that require significant additional components for actuation and control, and typically operate at much lower pressure.
The present invention is directed to overcoming the problems set forth above. It is desirable to have a simple, manually selectable combustion system capable of operating in either a diesel or a dual-fuel combustion mode. It is also desirable to have such a combustion system that is applicable to two-and/or four-stroke diesel engines and, desirably, may be easily retrofitted to existing diesel engines or readily applied to new dual-fuel engines. It is further desirable to have such a combustion system that uses a continuous duty pilot injection system to provide an ignition source for dual-fuel operation, and improved exhaust emissions when operating in the neat, or 100%, diesel combustion mode.